The present invention relates to turbines which are axially joined one to the other along their flowpath and particularly relates to a diffuser formed between and along the flowpath of axially joined turbines for reducing energy loss in large-scale turbulent mixing while recovering energy through diffusion of the fluid flow.
Turbines are sometimes connected by coupling their rotor shafts one to the other, as well as their flowpaths. For example, two axial steam turbines may be joined axially one to the other with the steam flow exiting the final stage of the first or upstream turbine entering the first stage of the second or downstream turbine. Typically, a cavity, which also forms part of the flowpath, is located between the turbines. With the rotating shaft and coupling exposed to the flowpath, the spinning of the shaft will entrain fluid and eject the fluid back into the flowpath. This is a phenomenon often referred to as windage loss and can create substantial energy loss through turbulent mixing in the cavity. The couplings between the shafts also present a protuberant surface to the flow along the flowpath from the one turbine to the other turbine through the cavity, causing losses due to flow separation. Other energy losses also occur in axially joined turbines. For example, the exit annulus of the upstream turbine typically has a different diameter and/or height than the entrance annulus of the downstream turbine. Since the flow cannot rapidly change direction from one annulus to the next, the flow will generally impinge upon other surfaces of the cavity, with consequent losses. Further, additional steam may be admitted to the flowpath, e.g., into the cavity, before the steam enters the downstream turbine. This intermediate steam admission creates a disturbance in the flowpath of the steam transitioning between the upstream and downstream turbines.
A prior effort to reduce losses from the rotating shaft included the provision of a generally cylindrical coupling cover overlying the cover and having an axis coincident with the axis of rotation of the turbines. While this addresses certain of the losses from the rotating shaft and coupling, it does not consider all of the loss mechanisms noted above. The cylindrical cover mitigates losses in the cavity but produces an energy loss itself and does not itself recover energy from the flowpath.
In accordance with a preferred embodiment of the present invention, there is provided apparatus for transitioning the flow from the upstream turbine to the downstream turbine and accommodating with reduced mixing losses a supplemental fluid flow admission into the cavity intermediate the upstream and downstream turbines. To accomplish the foregoing, there is provided a diffuser in the flowpath between the upstream and downstream turbines. An inner diffuser wall or coupling cover defines the inner diameter of the transitioning flowpath between the upstream and downstream turbines and extends between the final stage of the upstream turbine and the initial stage of the downstream turbine. The coupling cover is preferably in the form of a frustoconical section about an axis coincident with the axis of rotation of the turbine. Thus, the coupling cover overlies the coupling joining the rotor shafts to substantially minimize or preclude windage loss and flow separation due to protuberant surfaces which would otherwise be impacted by the fluid flow of the flowpath.
The diffuser also includes an outer diffuser wall which defines in part the outer margin of the flowpath between the upstream and downstream turbines. Like the inner coupling cover, the outer diffuser wall is preferably formed of a frustoconical section about the axis and is preferably cast as part of the outer turbine shell common to both turbines. The diffuser interposed between the exit annulus and entrance annulus of the upstream and downstream turbines, respectively, guides the fluid flow (steam) as it is being diffused. The diffuser therefore provides a smooth transition between the two turbines which reduces energy loss associated with the rotating shaft and coupling and misalignment between the exit and entrance annuli of the two turbines, while simultaneously increasing energy recovery through the use of a diffuser.
Supplemental fluid flow may be admitted into the flowpath cavity through an inlet intermediate the upstream and downstream turbines. The inlet is configured to turn the flow from essentially a radial direction to a flow direction having both axial and circumferentially directed components. When the supplemental admission flow meets the flowpath from the upstream turbine, the flow velocities and directions are such as to afford reduced mixing losses.
In a preferred embodiment according to the present invention, there is provided apparatus for coupling flowpaths of axially adjacent turbines to one another, comprising first and second turbines coupled axially to one another along a flowpath with fluid flow along a first flowpath portion along the first turbine exhausting from the first turbine and into a second flowpath portion along the second turbine, the turbines having respective rotors and a coupling between the first and second rotors for coupling the turbines to one another, an inner cover extending between a final stage of the first turbine and a first stage of the second turbine and extending about and overlying the coupling between the rotors to isolate the rotor coupling from the flowpath and present a substantially smooth transition of the fluid flow from the first flowpath portion of the first turbine to the second flowpath portion of the second turbine.
In a further preferred embodiment according to the present invention, there is provided apparatus for coupling turbines to one another, comprising first and second turbines coupled axially to one another and having a flowpath with fluid flow along a first flowpath portion exhausting from the first turbine and into a second flowpath portion of the second turbine, the turbines having respective rotors and a coupling between the first and second rotors for coupling the turbines to one another, an outer wall extending between a final stage of the first turbine and a first stage of the second turbine and about and overlying the flowpath between the first and second turbines to present a substantially smooth transition of the fluid flow from the first flowpath portion of the first turbine to the second flowpath portion of the second turbine.
In a further preferred embodiment according to the present invention, there is provided apparatus for coupling flowpaths of axially adjacent turbines to one another, comprising first and second turbines coupled axially to one another along a flowpath with fluid flow along a first flowpath portion along the first turbine exhausting from the first turbine through an exit annulus and into a second flowpath portion through an entry annulus to the second turbine, the turbines having respective rotors and a coupling between the first and second rotors for coupling the turbines to one another, annular wall portions extending from adjacent the exit annulus of the first turbine and radially outwardly of the coupling between the rotors forming a diffuser for conducting the fluid flow between the exit and entrance annuli and presenting a substantially smooth transition of the fluid flow from the first flowpath portion of the first turbine to the second flowpath portion of the second turbine.